1. Field of the Invention
The present invention relates to a television tuner for receiving a VHF band television signal.
2. Description of the Related Art
FIG. 3 shows a configuration of a conventional television tuner. An input tuning circuit 33 is coupled, via a filter 32 for removing undesired signals, to an input terminal 31 to which a VHF band television signal is inputted. The input tuning circuit 33 includes four inductors 33a to 33d connected in series and a varactor diode 33e connected in parallel with these inductors. Also, both ends of a switching diode 33f are connected in a high-frequency state between two nodes of these inductors 33a to 33d. The cathode of the switching diode 33f is DC-grounded (grounded for DC voltage) via a resistor 34 and is connected to a first terminal 36a of a band switching circuit 36 via a resistor 35. The anode thereof is connected to a second terminal 36b of the band switching circuit 36 via a resistor 37. The anode of the varactor diode 33e is grounded and tuning voltage is applied to the cathode thereof.
An output of the input tuning circuit 33 is provided to a field-effect transistor (dual gate FET) 38 constituting a high-frequency amplifier. A node of the cathode of the varactor diode 33e and the inductor 33d is coupled to a first gate which is an input terminal of the FET 38. A source of the FET 38 is grounded and a drain thereof is coupled to an interstage tuning circuit 39.
The interstage tuning circuit 39 is constituted by a double-tuned circuit. Provided on the primary side thereof are two inductors 39a and 39b connected in series, and the inductor 39a is connected to the drain of the FET 38. Also, provided on the secondary side are two inductors 39d and 39e connected in series via a DC cutting capacitor 39c, and the inductor 39d is coupled to a mixer (not shown). In addition, the inductors 39b and 39e are connected to a coupling inductor 39f, and the coupling inductor 39f is grounded in a high-frequency state via a DC cutting capacitor 39g. The power supply voltage B is applied to a node of the coupling inductor 39f and the DC cutting capacitor 39g via a resistor 40. Consequently, voltage is applied to the drain of the FET 38 via the resistor 40, the inductors 39a and 39b, and so forth.
A node of the inductors 39a and 39b on the primary side is connected to the anode of a switching diode 39i via a DC cutting capacitor 39h, and a node of the DC cutting capacitor 39c and the inductor 39e on the secondary side is connected to the anode of a switching diode 39k via a DC cutting capacitor 39j. The anode of the switching diode 39i is connected to the second terminal 36b via a resistor 41, and the anode of the switching diode 39k is connected to the second terminal 36b via a resistor 42. The cathodes of the switching diodes 39i and 39k are grounded via a resistor 43 and a DC cutting capacitor 44, and are connected to the first terminal 36a via a resistor 45.
In addition, provided on the primary side is a varactor diode 39m connected in parallel to the inductors 39a and 39b and so forth. Provided on the secondary side is a varactor diode 39n connected in parallel to the inductors 39d and 39e and so forth. The anodes of the varactor diodes 39m and 39n are grounded and tuning voltage is applied to their cathodes.
The band switching circuit 36 is for switching the input tuning circuit 33 and the interstage tuning circuit 39 so that they can tune into high and low VHF frequency band. When each of the tuning circuits 33 and 39 is tuned to the low band, high-level voltage (e.g., power supply voltage) is outputted to the first terminal 36a. When each of the tuning circuits 33 and 39 is tuned to the high band, high-level voltage is outputted to the second terminal 36b. The band switching circuit 36 is controlled by a channel-selection signal from channel-selection means (not shown).
In the configuration described above, when each of the tuning circuits 33 and 39 is tuned to the low band, voltage is applied to the cathodes of the switching diodes 33f, 39i, and 39k, but voltage is not applied to the anodes, so that the switching diodes 33f, 39i, and 39k are effectively deactivated (or turned off) by being in an unbiased state. Accordingly, every inductor becomes effective and a tuning frequency band becomes low. Further, the resistor 34 and the resistor 43 provide a current path for the activated switching diode 33f and the activated (i.e. turned on) switching diodes 39i and 39k, respectively, when each of the tuning circuits 33 and 39 is tuned to the high band. As a result, two inductors 33b and 33c are essentially removed from the input tuning circuit 33 by being effectively shorted by the activated switching diode 33f and the associated capacitor. Similarly, the effective inductance supplied by the four inductors 39b, 39a, 39d, and 39e and the coupling inductor 39f when the switching diodes 39i and 39k are turned on deviates from the inductance supplied when the switching diodes 39i and 39k are turned off in the interstage tuning circuit 39. Accordingly, the tuning frequency band of each of the tuning circuits 33 and 39 becomes high. In addition to altering the frequency using the switching diodes, the tuning frequency of each band may be changed by tuning voltage applied to each of the varactor diodes 33e, 39m, and 39n. 
In the configuration described above, distortion to an input signal may be generated when each of the switching diodes is turned off. That is, as shown in FIG. 4, each of the switching diodes is turned off by the voltage applied from the band switching circuit to each of the cathodes. However, voltage is not applied to each of the anodes at this time, and thus the anodes are left floating. Consequently, a reverse voltage is not applied to both ends of each of the switching diodes and the switching diodes are in an unbiased state. Thus, when a high level television signal is inputted, the switching diodes are rectified, generating distortion.
In addition, while resistor 43 of the interstage tuning circuit is provided to supply a current path when the switching diodes are turned on, current remains flowing through this resistor even when the switching diodes are turned off, increasing current consumption. Further, eight resistors are used in the conventional DC circuit for turning on or off each of the switching diodes. It would be beneficial to reduce the relatively large number of resistors and other components in the conventional DC circuit.